S. pyogenes (Group A Streptococcus; GAS) antigen GAS57, expressed as recombinant protein and purified from E. coli, induces protective activity against a lethal challenge with S. pyogenes in mice. However, GAS57 is a protease which cleaves and inactivates human chemokines such as interleukin-8 (IL-8) (Edwards et al., J Infectious Diseases 192, 783-90, 2005; Hidalgo-Grass et al., EMBO J 25, 4628-37, 2006). This property of GAS57 may hamper its use in a vaccine composition, due to possible side effects. Thus, there is a need in the art for GAS57 antigens which are unable to cleave human chemokines but which still maintain the ability to induce protection against S. pyogenes. There is also a need in the art for antibodies which specifically bind to GAS57 antigens and which impair the ability of GAS57 to cleave IL-8 and other substrates.